1. Field of the Invention
This invention relates to flame retardant resin compositions based on synthetic resins containing aromatic rings in a molecule such as aromatic polycarbonate resins and aromatic epoxy resins.
2. Prior Art
Flame retardant resin compositions are often used in various products such as electric and electronic parts, building members, automotive parts, and daily goods. These resin compositions are generally made flame retardant by adding organic halogen compounds optionally in admixture with antimony trioxide. However, these flame retardant resin compositions have the drawback that harmful halogen gases evolve upon combustion.
It is also known that resin compositions can be made flame retardant by adding silicone resins which do not evolve harmful gases.
JP-A 54-36365 discloses a flame retardant resin composition wherein a silicon resin containing at least 80% by weight of trifunctional siloxane units is added to an organic resin. With respect to the organic resin, no reference is made to aromatic polycarbonate resins and aromatic epoxy resins. With a focus on the melt processing of the organic resin, a relatively high molecular weight silicone resin substantially free of crosslinking functional radicals and having a softening point above room temperature is selected. On account of a relatively weak flame retardant effect, the silicone resin must be added in a large amount of about 10 to 300 parts by weight per 100 parts by weight of the organic resin, detracting from the properties of the organic resin.
JP-A 59-500099, 4-226159, and 7-33971 disclose flame retardant resin compositions having added thereto silicone resins consisting of monofunctional siloxane units and tetrafunctional siloxane units. JP-A 6-128434 discloses a flame retardant resin composition having added thereto a silicone resin containing vinyl-bearing siloxane units. In order to exert a satisfactory flame retardant effect, all these compositions, however, require to increase the amount of silicone resin added or to additionally use inorganic fillers such as aluminum hydroxide, halogens or phosphorus compounds.
In most systems with silicone resins added, the amount of silicone resin added must be increased in order to achieve a satisfactory flame retardant effect, but at the sacrifice of the moldability and mechanical strength of resin compositions. Efforts have been made to develop silicone resin additives having greater flame retardant effect or additives capable of cooperating with silicone resins to improve the flame retardant effect.
JP-A 8-176425 discloses a flame retardant resin composition comprising a polycarbonate resin, an epoxy-bearing organopolysiloxane, and an alkali metal salt of an organic sulfonic acid. JP-A 8-176427 discloses a flame retardant resin composition comprising a polycarbonate resin, a polycarbonate resin modified with a phenolic hydroxyl-bearing organopolysiloxane, and an organic alkali metal salt. Further, JP-A 9-169914 discloses a composition wherein a petroleum heavy oil or pitch is combined with a silicone compound for improving flame retardance. These silicone resins having special organic functional radicals are expensive because of the complication of their preparation process, but do not achieve a sufficient flame retardant effect to compensate for the increased cost. In this regard, a further improvement is desired.
Also for improving the thermal oxidation resistance of polycarbonate resins, it is known effective to add a silicone resin having alkoxy functional radicals which can be introduced at a relative low cost. JP-A 54-102352 discloses a thermoplastic resin composition having added thereto a silicone resin containing alkoxy radicals as shown below. ##STR1##
x is an integer of 2 to 10. PA1 (A) 100 parts by weight of a synthetic resin containing aromatic rings in a molecule, and PA1 (B) 0.1 to 10 parts by weight of an organopolysiloxane containing phenyl radicals as essential substituents attached to silicon atoms and siloxane units represented by R.sup.1 R.sup.2 R.sup.3 SiO.sub.1/2 wherein R.sup.1, R.sup.2, and R.sup.3 are independently selected from substituted or unsubstituted monovalent hydrocarbon radicals of 1 to 6 carbon atoms, the contents of alkoxy radicals and hydroxyl radicals available as Si--OH radicals in a molecule each being less than 2% by weight, the organopolysiloxane having a weight average molecular weight of up to 2,000.
In general, siloxanes having a greater alkoxy content tend to form a network and provide a greater thermal oxidation resistance. The former silicone resin provides insufficient flame retardance because of the lack of phenyl radicals. The latter phenyl-bearing, low molecular weight organosiloxane is also insufficient in imparting flame retardance because of a substantial loss of effective components through vaporization by heat during melt processing or combustion.
JP-A 1-318069 discloses a powdery polymer mixture of an alkoxy or hydroxyl radical-bearing silicone resin and a thermoplastic polymer. JP-A 6-306265 discloses a flame retardant polycarbonate resin composition comprising a polycarbonate and an organic siloxane having alkoxy, phenyl and vinyl radicals introduced therein. The former requires the addition of halogen and phosphorus compounds because the silicone resin alone imparts insufficient flame retardance. In the latter composition, the siloxane contains as the alkoxy radicals lower alkoxy radicals such as methoxy which are so sensitive to hydrolysis that they form alcohols, detracting from the transparency of the resin composition.
In the recent years, there is an increasing demand to recycle various plastic materials for reuse. The above-mentioned silicone compounds having reactive radicals such as alkoxy, hydroxyl or epoxy radicals change their molecular structure during molding of resin compositions or during service of molded resin parts, due to the interaction between silicone compounds or with another component in the resin composition and under the influence of the service environment. As a result, many recycled materials become difficult to mold and some materials are even impossible to recycle.
JP-A 10-139964 discloses a flame retardant resin composition to which is added a silicone resin composed of di- and trifunctional siloxane units and having a weight average molecular weight of 10,000 to 270,000. It is described that a monofunctional siloxane unit represented by R'.sub.3 SiO.sub.1/2 is introduced as a terminal radical wherein hydroxyl and/or alkoxy radicals preferably account for less than 10 mol % of the R' radicals. However, the contents of hydroxyl and alkoxy radicals in the molecule are not specified. Since a high molecular weight silicone resin is used, the composition is still insufficient in moldability and flame retardance.
There is a demand to have a flame retardant resin composition comprising an aromatic ring-bearing synthetic resin such as an aromatic polycarbonate resin or aromatic epoxy resin and a minor amount of a flame retardant whereby molded resin parts having improved flame retardance can be produced while maintaining the physical properties and optical transparency of the synthetic resin and the resin parts can be recycled for reuse.